In LTE-Advanced, an enhanced version of 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), hybrid transmission, in which switching is performed between SC-FDMA (Single Carrier-Frequency Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access) in an uplink, has been investigated (see Non-Patent Literature 1, for example).
An advantage of OFDMA is that more flexible frequency resource allocation is possible than in the case of SC-FDMA, and therefore frequency scheduling gain is obtained. Thus, OFDMA enables improved throughput performance. On the other hand, an advantage of SC-FDMA is that PAPR (Peak-to-Average Power Ratio) indicating a ratio of peak to average power of a transmission signal, and CM (Cubic Metric), are smaller than in the case of OFDMA. Consequently, if power amplifiers with the same maximum transmission power specification are used for SC-FDMA and OFDMA, power amplifier back-off necessary for transmitting a transmission signal without distortion can be made smaller in the case of SC-FDMA. Thus, SC-FDMA can increase actually transmissible maximum power, enabling improved coverage performance.
Hybrid transmission enables the respective above advantages to be obtained by switching adaptively between SC-FDMA and OFDMA according to the communication environment of a mobile station.
Investigation has been carried out into having control of switching between SC-FDMA and OFDMA performed by a base station based on power headroom (hereinafter referred to as “PHR”) information indicating a margin of power (possible increase in power) of the transmission power of a mobile station. Non-Patent Literature 1 describes applying OFDMA to a mobile station with a PHR margin because transmission power is low, and applying SC-FDMA to a mobile station with no PHR margin because transmission power is high.
The PHR definition and transmitting method investigated in LTE will now be described. With LTE, a mobile station transmits PHR by means of a data channel in order for PHR to be used when a base station performs transmission power control, MCS (Modulation and channel Coding Scheme) control, and transmission bandwidth control. Non-Patent Literature 2 includes a PHR definition and PHR transmission conditions according to equation 1.[1]PHR=10 log10(PMAX)−(10 log10M+P0+αPL+ΔMCS+f(Δi))  (Equation 1)
Here, PHR indicates power headroom [dB], PMAX indicates maximum transmission power [mW], M indicates an allocated number of frequency resource blocks, P0 indicates an offset (a parameter signaled from a base station) [dB], PL indicates a path loss level [dB], a indicates a weighting coefficient for path loss, ΔMCS indicates an MCS-dependent offset, and f(Δi) indicates a transmission power control value subject to closed loop control.
When a mobile station moves, path loss occurs, and therefore PHR fluctuates temporally. Consequently, it is necessary for a mobile station to report PHR to a base station at a predetermined period and when a predetermined condition is satisfied. Non-Patent Literature 2 discloses reporting of PHR to a base station by a mobile station if PHR is less than Y [dB] or if path loss changes by X [dB], and also describes reporting of PHR at N-frame intervals (where Y, X, and N are parameters).
Non-Patent Literature 3 describes PHR being transmitted as data MAC (Medium Access Control) information by means of a data channel (in LTE, a PUSCH (Physical Uplink Shared Channel)).
FIG. 1 shows the relationship between PHR and throughput performance. In FIG. 1, the solid line indicates an OFDMA characteristic, and the dotted line indicates an SC-FDMA characteristic. As shown in FIG. 1, when there is a PHR margin (PHR is large), OFDMA provides better throughput performance than SC-FDMA due to such advantages as a frequency scheduling effect. On the other hand, when there is no PHR margin (PHR is small), OFDMA throughput performance and SC-FDMA throughput performance are reversed, and SC-FDMA throughput performance is better.
The reason for the degradation of OFDMA throughput performance is that, when PHR becomes small, CM and PAPR power amplifier back-off becomes necessary, and a mobile station cannot transmit data at a transmission power level specified by a base station. On the other hand, with SC-FDMA, the CM and PAPR are smaller than with OFDMA, and back-off necessary for a power amplifier is also smaller, and therefore a mobile station can transmit data at a transmission power level specified by a base station, and degradation of throughput performance can be suppressed. Thus, switching between OFDMA and SC-FDMA at PHR at which throughput performances coincide is ideal.
Also, with LTE-Advanced, applying precoding in an uplink (here, meaning stream multiplexing prior to transmission power amplification) has been investigated. Below, application of precoding is referred to as “precoding ON”, and non-application of precoding is referred to as “precoding OFF”.
An advantage of performing directional transmission with precoding ON is that the reception SINR can be improved. However, with OFDMA, the CM of a transmission signal is quite large to begin with, and therefore there is hardly any variation in the CM between precoding ON and precoding OFF, whereas with OFDMA, a difference in the CM arises between precoding ON and precoding OFF.
FIG. 2 shows the results of a computer simulation carried out by the present inventors. As can be seen from FIG. 2, with OFDMA there is hardly any difference in the CM between precoding ON and precoding OFF, whereas with SC-FDMA there is a difference of 1.3 dB in the CM between precoding OFF and precoding ON. Consequently, with SC-FDMA, actually transmissible maximum power can be increased, enabling improved coverage performance. Thus, with SC-FDMA, it is possible to conceive of a base station controlling precoding ON/OFF switching based on PHR information.
FIG. 3 shows the relationship between PHR and throughput performance in SC-FDMA. In FIG. 3, the solid line indicates a precoding ON characteristic, and the dotted line indicates a precoding OFF characteristic. As shown in FIG. 3, when there is a PHR margin (PHR is large), throughput performance is better with precoding ON than with precoding OFF due to such advantages as an improvement in the SINR through directional transmission. On the other hand, when there is no PHR margin (PHR is small), precoding ON throughput performance and precoding OFF throughput performance are reversed, and precoding OFF throughput performance is better.
The reason for the degradation of precoding ON throughput performance is that, when PHR becomes small, CM and PAPR power amplifier back-off becomes necessary, and a mobile station cannot transmit data at a transmission power level specified by a base station. On the other hand, with precoding OFF, the CM and PAPR are smaller than with precoding ON, and back-off necessary for a power amplifier is also smaller, and therefore a mobile station can transmit data at a transmission power level specified by a base station, and degradation of throughput performance can be suppressed. Thus, switching between precoding ON and OFF in SC-FDMA at PHR at which throughput performances coincide is ideal.
Also, with LTE-Advanced, switching between single-carrier transmission and multi-carrier transmission in an uplink using a DFT-s-OFDM-with-SDC method has been investigated (see Non-Patent Literature 4, for example).
FIG. 4 is a block diagram showing a general configuration of a DFT-s-OFDM-with-SDC-type transmitting apparatus. As shown in FIG. 4, this transmitting apparatus performs DFT (Discrete Fourier Transform) processing on a data signal, and maps a post-DFT data signal to the frequency domain. A mapped data signal undergoes IFFT (Inverse Fast Fourier Transform) processing and CP (Cyclic Prefix) addition, and is then transmitted. Having the subcarrier mapping section shown in FIG. 4 control the data signal frequency-domain mapping method enables switching between single-carrier transmission and multi-carrier transmission. Specifically, if the number of frequency-domain data divisions (hereinafter referred to as SDs (Spectrum Divisions)) is 1, single-carrier transmission is used, and if the number of SDs≧2, multi-carrier transmission is used. By controlling the number of SDs according to the communication environment of a mobile station, a base station can switch adaptively between single-carrier transmission and multi-carrier transmission.
An advantage of a DFT-s-OFDM-with-SDC method is that frequency scheduling gain can be obtained by increasing the degree of freedom of transmission data frequency allocation by increasing the number of SDs. However, as the number of SDs is increased, the CM also increases.
FIG. 5 shows the results of a computer simulation carried out by the present inventors to evaluate the relationship between the number of SDs and the CM for a DFT-s-OFDM-with-SDC method. As can be seen from FIG. 5, the CM increases as the number of SDs increases. Consequently, a smaller number of SDs enables actually transmissible maximum power to be increased, enabling improved coverage performance. Thus, with a DFT-s-OFDM-with-SDC method, it is possible to conceive of a base station controlling switching between a larger and smaller number of SDs based on PHR information.
FIG. 6 shows the relationship between PHR and throughput performance for a DFT-s-OFDM-with-SDC method. In FIG. 6, the solid line indicates a characteristic for a larger number of SDs (for example, number of SDs≧3), and the dotted line indicates a characteristic for a smaller number of SDs (for example, number of SDs<3). As shown in FIG. 6, when there is a PHR margin (PHR is large), throughput performance is better with a larger number of SDs than with a smaller number of SDs due to such advantages as an improvement in frequency scheduling gain through an improvement in the degree of freedom of transmission data allocation. On the other hand, when there is no PHR margin (PHR is small), throughput performance with a larger number of SDs and throughput performance with a smaller number of SDs are reversed, and throughput performance with a smaller number of SDs is better.
The reason for the degradation of throughput performance with a larger number of SDs is that, when PHR becomes small, CM and PAPR power amplifier back-off becomes necessary, and a mobile station cannot transmit data at a transmission power level specified by a base station. On the other hand, with a smaller number of SDs, the CM and PAPR are smaller than with a larger number of SDs, and back-off necessary for a power amplifier is also smaller, and therefore a mobile station can transmit data at a transmission power level specified by a base station, and degradation of throughput performance can be suppressed. Thus, performing switching between a larger and smaller number of SDs at PHR at which throughput performances coincide is ideal.